1. Field of the Invention
The present invention is generally related to offloading cable modem termination system (CMTS) software by using hardware to reassemble fragmented frames or packets from a cable modem.
2. Related Art
The importance to the modem economy of rapid data access and exchange cannot be overstated. This explains the exponentially increasing popularity of the data access and exchange via cable networks (including coaxial cable or Hybrid fiber coaxial cable), the Internet, intranets, wireless networks, satellites, optical and so forth (i.e., communication mediums). Rapid data access and exchange is partly dependent upon how efficiently bandwidth is allocated to a data provider in order for the data provider to transfer the requested data to a user via one of the communication mediums mentioned above.
One very desirable solution for rapid data access and exchange is via cable networks and cable modems. Cable modems provide asynchronous communications on cable networks. In general, a user connects a cable modem to the TV outlet for his or her cable TV, and the cable TV operator connects a cable modem termination system (“CMTS”) in the operator's headend. The CMTS is a central device for connecting the cable network to a data network like the Internet. The CMTS is a central distribution point for a cable system. Data flows “downstream” from the CMTS to the cable modem (i.e., downstream communication). Alternatively, data flows “upstream” from the cable modem to the CMTS (i.e., upstream communication).
A common cable modem standard today is the Data Over Cable Service Interface Specification (“DOCSIS”). DOCSIS defines technical specifications for both cable modems and CMTS.
In general, a cable modem forwards or provides data via asynchronous communications on cable networks. The cable modem receives data from a user that needs to be transferred via a cable network. For many types of data, in order for the cable modem to transfer the data via a cable network it must request that the CMTS grant to it the necessary bandwidth to send a data packet or frame. At times the CMTS only grants the cable modem enough bandwidth to send part of the packet. In this case, the cable modem sends as much of the packet as will fit in the bandwidth grant, along with a special “fragment header” indicating that it is sending a fragment and including a sequence number for the fragment and flags indicating when the fragment is the first or last fragment of the packet. With fragmentation, a cyclic redundancy check (CRC) is appended to the fragment that covers only the bytes in the fragment.
Subsequent bandwidth grants from the CMTS may be large enough to fill all or part of the remainder of the packet. In the case where only part of the remainder of the packet fits in the granted bandwidth, the cable modem sends another fragment as above, and so on until the entire remainder of the packet fits in a bandwidth grant. When the cable modem sends a final fragment in the packet, the cable modem sets a flag indicating that this fragment is the last fragment in the packet.
At the CMTS, fragments sent by the cable modem must be reassembled to reconstruct the original frame or packet by the CMTS software. The reassembly operation involves removing the special fragment header and CRC, combining the data from each fragment in the correct order, maintaining the session state, and using the session state to check for error conditions such as lost fragments or incorrect sequence numbers. These functions must be performed separately for each cable modem (i.e., data provider) and for each flow from a particular cable modem.
The reassembly of fragmented frames is handled by software running on an embedded CPU in the CMTS. This requires the CMTS software to do detailed parsing of every frame, even non-fragmented ones (since arrival of a non-fragmented frame can affect the state of a fragmentation session). This burden on software reduces system performance and increases cost by requiring a higher performance CPU or multiple CPUs.